MAP kinase and cAMP signaling regulate infection structure formation and pathogenic growth in the rice blast fungus Magnaporthe grisea

Appressorium-mediated plant infection by Magnaporthe oryzae is regulated by a Pmk1-dependent hierarchical transcriptional network

Rice blast is a devastating disease caused by the fungal pathogen Magnaporthe oryzae that threatens rice production around the world. The fungus produces a specialized infection cell, called the appressorium, that enables penetration through the plant cell wall in response to surface signals from the rice leaf. The underlying biology of plant infection, including the regulation of appressorium formation, is not completely understood. Here we report the identification of a network of temporally coregulated transcription factors that act downstream of the Pmk1 mitogen-activated protein kinase pathway to regulate gene expression during appressorium-mediated plant infection. We show that this tiered regulatory mechanism involves Pmk1-dependent phosphorylation of the Hox7 homeobox transcription factor, which regulates genes associated with induction of major physiological changes required for appressorium development-including cell-cycle control, autophagic cell death, turgor generation and melanin biosynthesis-as well as controlling a additional set of virulence-associated transcription factor-encoding genes. Pmk1-dependent phosphorylation of Mst12 then regulates gene functions involved in septin-dependent cytoskeletal re-organization, polarized exocytosis and effector gene expression, which are necessary for plant tissue invasion. Identification of this regulatory cascade provides new potential targets for disease intervention.

Host-Induced Gene Silencing of a Multifunction Gene Sscnd1 Enhances Plant Resistance Against Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a devastating necrotrophic fungal pathogen and has a substantial economic impact on crop production worldwide. Magnaporthe appressoria-specific (MAS) proteins have been suggested to be involved in the appressorium formation in Magnaporthe oryzae. Sscnd1, an MAS homolog gene, is highly induced at the early infection stage of S. sclerotiorum. Knock-down the expression of Sscnd1 gene severely reduced the virulence of S. sclerotiorum on intact rapeseed leaves, and their virulence was partially restored on wounded leaves. The Sscnd1 gene-silenced strains exhibited a defect in compound appressorium formation and cell integrity. The instantaneous silencing of Sscnd1 by tobacco rattle virus (TRV)-mediated host-induced gene silencing (HIGS) resulted in a significant reduction in disease development in tobacco. Three transgenic HIGS Arabidopsis lines displayed high levels of resistance to S. sclerotiorum and decreased Sscnd1 expression. Production of specific Sscnd1 siRNA in transgenic HIGS Arabidopsis lines was confirmed by stem-loop qRT-PCR. This study revealed that the compound appressorium-related gene Sscnd1 is required for cell integrity and full virulence in S. sclerotiorum and that Sclerotinia stem rot can be controlled by expressing the silencing constructs of Sscnd1 in host plants.

Endosomal sorting complexes required for transport-0 (ESCRT-0) are essential for fungal development, pathogenicity, autophagy and ER-phagy in Magnaporthe oryzae

Magnaporthe oryzae is an important plant pathogen that causes rice blast. Hse1 and Vps27 are components of ESCRT-0 involved in the multivesicular body (MVB) sorting pathway and biogenesis. To date, the biological functions of ESCRT-0 in M. oryzae have not been determined. In this study, we identified and characterized Hse1 and Vps27 in M. oryzae. Disruption of MoHse1 and MoVps27 caused pleiotropic defects in growth, conidiation, sexual development and pathogenicity, thereby resulting in loss of virulence in rice and barley leaves. Disruption of MoHse1 and MoVps27 triggered increased lipidation of MoAtg8 and degradation of GFP-MoAtg8, indicating that ESCRT-0 is involved in the regulation of autophagy. ESCRT-0 was determined to interact with coat protein complex II (COPII), a regulator functioning in homeostasis of the endoplasmic reticulum (ER homeostasis), and disruption of MoHse1 and MoVps27 also blocked activation of the unfolded protein response (UPR) and autophagy of the endoplasmic reticulum (ER-phagy). Overall, our results indicate that ESCRT-0 plays critical roles in regulating fungal development, virulence, autophagy and ER-phagy in M. oryzae.

Prey sensing and response in a nematode-trapping fungus is governed by the MAPK pheromone response pathway

Detection of surrounding organisms in the environment plays a major role in the evolution of interspecies interactions, such as predator-prey relationships. Nematode-trapping fungi (NTF) are predators that develop specialized trap structures to capture, kill, and consume nematodes when food sources are limited. Despite the identification of various factors that induce trap morphogenesis, the mechanisms underlying the differentiation process have remained largely unclear. Here, we demonstrate that the highly conserved pheromone-response MAPK pathway is essential for sensing ascarosides, a conserved molecular signature of nemaotdes, and is required for the predatory lifestyle switch in the NTF Arthrobotrys oligospora. Gene deletion of STE7 (MAPKK) and FUS3 (MAPK) abolished nematode-induced trap morphogenesis and conidiation and impaired the growth of hyphae. The conserved transcription factor Ste12 acting downstream of the pheromone-response pathway also plays a vital role in the predation of A. oligospora. Transcriptional profiling of a ste12 mutant identified a small subset of genes with diverse functions that are Ste12 dependent and could trigger trap differentiation. Our work has revealed that A. oligospora perceives and interprets the ascarosides produced by nematodes via the conserved pheromone signaling pathway in fungi, providing molecular insights into the mechanisms of communication between a fungal predator and its nematode prey.

Emerging Roles of Posttranslational Modifications in Plant-Pathogenic Fungi and Bacteria

Posttranslational modifications (PTMs) play crucial roles in regulating protein function and thereby control many cellular processes and biological phenotypes in both eukaryotes and prokaryotes. Several recent studies illustrate how plant fungal and bacterial pathogens use these PTMs to facilitate development, stress response, and host infection. In this review, we discuss PTMs that have key roles in the biological and infection processes of plant-pathogenic fungi and bacteria. The emerging roles of PTMs during pathogen-plant interactions are highlighted. We also summarize traditional tools and emerging proteomics approaches for PTM research. These discoveries open new avenues for investigating the fundamental infection mechanisms of plant pathogens and the discovery of novel strategies for plant disease control.

Regulation of biotic interactions and responses to abiotic stresses by MAP kinase pathways in plant pathogenic fungi

Like other eukaryotes, fungi use MAP kinase (MAPK) pathways to mediate cellular changes responding to external stimuli. In the past two decades, three well-conserved MAP kinase pathways have been characterized in various plant pathogenic fungi for regulating responses and adaptations to a variety of biotic and abiotic stresses encountered during plant infection or survival in nature. The invasive growth (IG) pathway is homologous to the yeast pheromone response and filamentation pathways. In plant pathogens, the IG pathway often is essential for pathogenesis by regulating infection-related morphogenesis, such as appressorium formation, penetration, and invasive growth. The cell wall integrity (CWI) pathway also is important for plant infection although the infection processes it regulates vary among fungal pathogens. Besides its universal function in cell wall integrity, it often plays a minor role in responses to oxidative and cell wall stresses. Both the IG and CWI pathways are involved in regulating known virulence factors as well as effector genes during plant infection and mediating defenses against mycoviruses, bacteria, and other fungi. In contrast, the high osmolarity growth (HOG) pathway is dispensable for virulence in some fungi although it is essential for plant infection in others. It regulates osmoregulation in hyphae and is dispensable for appressorium turgor generation. The HOG pathway also plays a major role for responding to oxidative, heat, and other environmental stresses and is overstimulated by phenylpyrrole fungicides. Moreover, these three MAPK pathways crosstalk and coordinately regulate responses to various biotic and abiotic stresses. The IG and CWI pathways, particularly the latter, also are involved in responding to abiotic stresses to various degrees in different fungal pathogens, and the HOG pathway also plays a role in interactions with other microbes or fungi. Furthermore, some infection processes or stress responses are co-regulated by MAPK pathways with cAMP or Ca2+/CaM signaling. Overall, functions of individual MAP kinase pathways in pathogenesis and stress responses have been well characterized in a number of fungal pathogens, showing the conserved genetic elements with diverged functions, likely by rewiring transcriptional regulatory networks. In the near future, applications of genomics and proteomics approaches will likely lead to better understanding of crosstalk among the MAPKs and with other signaling pathways as well as roles of MAPKs in defense against other microbes (biotic interactions).

Comparative Transcriptome and Endophytic Bacterial Community Analysis of Morchella conica SH

The precious rare edible fungus Morchella conica is popular worldwide for its rich nutrition, savory flavor, and varieties of bioactive components. Due to its high commercial, nutritional, and medicinal value, it has always been a hot spot. However, the molecular mechanism and endophytic bacterial communities in M. conica were poorly understood. In this study, we sequenced, assembled, and analyzed the genome of M. conica SH. Transcriptome analysis reveals significant differences between the mycelia and fruiting body. As shown in this study, 1,329 and 2,796 genes were specifically expressed in the mycelia and fruiting body, respectively. The Gene Ontology (GO) enrichment showed that RNA polymerase II transcription activity-related genes were enriched in the mycelium-specific gene cluster, and nucleotide binding-related genes were enriched in the fruiting body-specific gene cluster. Further analysis of differentially expressed genes in different development stages resulted in finding two groups with distinct expression patterns. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment displays that glycan degradation and ABC transporters were enriched in the group 1 with low expressed level in the mycelia, while taurine and hypotaurine metabolismand tyrosine metabolism-related genes were significantly enriched in the group 2 with high expressed level in mycelia. Moreover, a dynamic shift of bacterial communities in the developing fruiting body was detected by 16S rRNA sequencing, and co-expression analysis suggested that bacterial communities might play an important role in regulating gene expression. Taken together, our study provided a better understanding of the molecular biology of M. conica SH and direction for future research on artificial cultivation.

Diversity and Function of Appressoria

Endophytic, saprobic, and pathogenic fungi have evolved elaborate strategies to obtain nutrients from plants. Among the diverse plant-fungi interactions, the most crucial event is the attachment and penetration of the plant surface. Appressoria, specialized infection structures, have been evolved to facilitate this purpose. In this review, we describe the diversity of these appressoria and classify them into two main groups: single-celled appressoria (proto-appressoria, hyaline appressoria, melanized (dark) appressoria) and compound appressoria. The ultrastructure of appressoria, their initiation, their formation, and their function in fungi are discussed. We reviewed the molecular mechanisms regulating the formation and function of appressoria, their strategies to evade host defenses, and the related genomics and transcriptomics. The current review provides a foundation for comprehensive studies regarding their evolution and diversity in different fungal groups.

A putative PKA phosphorylation site S227 in MoSom1 is essential for infection-related morphogenesis and pathogenicity in Magnaporthe oryzae

In the rice blast fungus Magnaporthe oryzae, the cAMP signalling pathway plays a critical role in regulating leaf surface recognition and the initiation of appressorium development. Direct downstream targets of the cAMP signalling pathway are, however, not well-characterised. The MoSom1 protein functions downstream of the cAMP dependent protein kinase A (cAMP-PKA) and is essential for infection-related morphogenesis and pathogenicity. In this study, we show that mutation of a putative PKA phosphorylation site in MoSom1 is essential for its role in appressorium differentiation and pathogenicity in M. oryzae. Mutation of serine 227 in MoSom1 by deletion or serine (S) substitution to alanine (A), valine (V) or tyrosine (Y), resulted in defects of conidiation, appressorium-like structure formation and fungal pathogenicity. Western blot analysis confirmed that S227 in MoSom1 is a putative PKA phosphorylation site. Furthermore, a ΔMosom1 mutant showed reduced expression of PMK1 and was defective in Pmk1 phosphorylation, indicating that the Pmk1 mitogen-activated protein kinase (MAPK) acts downstream of MoSom1 in M. oryzae. We conclude that the cAMP-PKA pathway may regulate the Pmk1 MAPK pathway through MoSom1 during rice infection by the blast fungus. TAKE AWAYS: S227 is crucial for MoSom1 function in M. oryzae. S227 in MoSom1 was identified as a putative PKA phosphorylation site in M. oryzae. S227 is essential for infection-related morphogenesis and pathogenicity in M. oryzae.

CcPmk1 is a regulator of pathogenicity in Cytospora chrysosperma and can be used as a potential target for disease control

Fus3/Kss1, also known as Pmk1 in several pathogenic fungi, is a component of the mitogen-activated protein kinase (MAPK) signalling pathway that functions as a regulator in fungal development, stress response, mating, and pathogenicity. Cytospora chrysosperma, a notorious woody plant-pathogenic fungus, causes canker disease in many species, and its Pmk1 homolog, CcPmk1, is required for fungal development and pathogenicity. However, the global regulation network of CcPmk1 is still unclear. In this study, we compared transcriptional analysis between a CcPmk1 deletion mutant and the wild type during the simulated infection process. A subset of transcription factor genes and putative effector genes were significantly down-regulated in the CcPmk1 deletion mutant, which might be important for fungal pathogenicity. Additionally, many tandem genes were found to be regulated by CcPmk1. Eleven out of 68 core secondary metabolism biosynthesis genes and several gene clusters were significantly down-regulated in the CcPmk1 deletion mutant. GO annotation of down-regulated genes showed that the ribosome biosynthesis-related processes were over-represented in the CcPmk1 deletion mutant. Comparison of the CcPmk1-regulated genes with the Pmk1-regulated genes from Magnaporthe oryzae revealed only a few overlapping regulated genes in both CcPmk1 and Pmk1, while the enrichment GO terms in the ribosome biosynthesis-related processes were also found. Subsequently, we calculated that in vitro feeding artificial small interference RNAs of CcPmk1 could silence the target gene, resulting in inhibited fungal growth. Furthermore, silencing of BcPmk1 in Botrytis cinerea with conserved CcPmk1 and BcPmk1 fragments could significantly compromise fungal virulence using the virus-induced gene silencing system in Nicotiana benthamiana. These results suggest that CcPmk1 functions as a regulator of pathogenicity and can potentially be designed as a target for broad-spectrum disease control, but unintended effects on nonpathogenic fungi need to be avoided.

Prp19-associated splicing factor Cwf15 regulates fungal virulence and development in the rice blast fungus

The splicing factor Cwf15 is an essential component of the Prp19-associated component of the spliceosome and regulates intron splicing in several model species, including yeasts and human cells. However, the roles of Cwf15 remain unexplored in plant pathogenic fungi. Here, we report that MoCWF15 in the rice blast fungus Magnaporthe oryzae is non-essential to viability and important to fungal virulence, growth and conidiation. MoCwf15 contains a putative nuclear localization signal (NLS) and is localized into the nucleus. The NLS sequence but not the predicted phosphorylation site or two sumoylation sites was essential for the biological functions of MoCwf15. Importantly, MoCwf15 physically interacted with the Prp19-associated splicing factors MoCwf4, MoSsa1 and MoCyp1, and negatively regulated protein accumulations of MoCyp1 and MoCwf4. Furthermore, with the deletion of MoCWF15, aberrant intron splicing occurred in near 400 genes, 20 of which were important to the fungal development and virulence. Taken together, MoCWF15 regulates fungal growth and infection-related development by modulating the intron splicing efficiency of a subset of genes in the rice blast fungus.

Chitosan inhibits septin-mediated plant infection by the rice blast fungus Magnaporthe oryzae in a protein kinase C and Nox1 NADPH oxidase-dependent manner

Chitosan is a partially deacetylated linear polysaccharide composed of β-1,4-linked units of d-glucosamine and N-acetyl glucosamine. As well as a structural component of fungal cell walls, chitosan is a potent antifungal agent. However, the mode of action of chitosan is poorly understood. Here, we report that chitosan is effective for control of rice blast disease. Chitosan application impairs growth of the blast fungus Magnaporthe oryzae and has a pronounced effect on appressorium-mediated plant infection. Chitosan inhibits septin-mediated F-actin remodelling at the appressorium pore, thereby preventing repolarization of the infection cell. Chitosan causes plasma membrane permeabilization of M. oryzae and affects NADPH oxidase-dependent synthesis of reactive oxygen species, essential for septin ring formation and fungal pathogenicity. We further show that toxicity of chitosan to M. oryzae requires the protein kinase C-dependent cell wall integrity pathway, the Mps1 mitogen-activated protein kinase and the Nox1 NADPH oxidase. A conditionally lethal, analogue (PP1)-sensitive mutant of Pkc1 is partially remediated for growth in the presence of chitosan, while ∆nox1 mutants increase their glucan : chitin cell wall ratio, rendering them resistant to chitosan. Taken together, our data show that chitosan is a potent fungicide which requires the cell integrity pathway, disrupts plasma membrane function and inhibits septin-mediated plant infection.

Investigating the cell and developmental biology of plant infection by the rice blast fungus Magnaporthe oryzae

Magnaporthe oryzae is the causal agent of rice blast disease, the most widespread and serious disease of cultivated rice. Live cell imaging and quantitative 4D image analysis have provided new insight into the mechanisms by which the fungus infects host cells and spreads rapidly in plant tissue. In this video review article, we apply live cell imaging approaches to understanding the cell and developmental biology of rice blast disease. To gain entry to host plants, M. oryzae develops a specialised infection structure called an appressorium, a unicellular dome-shaped cell which generates enormous turgor, translated into mechanical force to rupture the leaf cuticle. Appressorium development is induced by perception of the hydrophobic leaf surface and nutrient deprivation. Cargo-independent autophagy in the three-celled conidium, controlled by cell cycle regulation, is essential for appressorium morphogenesis. Appressorium maturation involves turgor generation and melanin pigment deposition in the appressorial cell wall. Once a threshold of turgor has been reached, this triggers re-polarisation which requires regulated generation of reactive oxygen species, to facilitate septin GTPase-dependent cytoskeletal re-organisation and re-polarisation of the appressorium to form a narrow, rigid penetration peg. Infection of host tissue requires a further morphogenetic transition to a pseudohyphal-type of growth within colonised rice cells. At the same time the fungus secretes an arsenal of effector proteins to suppress plant immunity. Many effectors are secreted into host cells directly, which involves a specific secretory pathway and a specialised structure called the biotrophic interfacial complex. Cell-to-cell spread of the fungus then requires development of a specialised structure, the transpressorium, that is used to traverse pit field sites, allowing the fungus to maintain host cell membrane integrity as new living plant cells are invaded. Thereafter, the fungus rapidly moves through plant tissue and host cells begin to die, as the fungus switches to necrotrophic growth and disease symptoms develop. These morphogenetic transitions are reviewed in the context of live cell imaging studies.

Unfolded Protein Response and Scaffold Independent Pheromone MAP Kinase Signaling Control Verticillium dahliae Growth, Development, and Plant Pathogenesis

Differentiation, growth, and virulence of the vascular plant pathogen Verticillium dahliae depend on a network of interconnected cellular signaling cascades. The transcription factor Hac1 of the endoplasmic reticulum-associated unfolded protein response (UPR) is required for initial root colonization, fungal growth, and vascular propagation by conidiation. Hac1 is essential for the formation of microsclerotia as long-time survival resting structures in the field. Single endoplasmic reticulum-associated enzymes for linoleic acid production as precursors for oxylipin signal molecules support fungal growth but not pathogenicity. Microsclerotia development, growth, and virulence further require the pheromone response mitogen-activated protein kinase (MAPK) pathway, but without the Ham5 scaffold function. The MAPK phosphatase Rok1 limits resting structure development of V.dahliae, but promotes growth, conidiation, and virulence. The interplay between UPR and MAPK signaling cascades includes several potential targets for fungal growth control for supporting disease management of the vascular pathogen V.dahliae.

The Membrane-Bound Protein, MoAfo1, Is Involved in Sensing Diverse Signals from Different Surfaces in the Rice Blast Fungus

To establish an infection, fungal pathogens must recognize diverse signals from host surfaces. The rice blast fungus, Magnaporthe oryzae, is one of the best models studying host-pathogen interactions. This fungus recognizes physical or chemical signals from the host surfaces and initiates the development of an infection structure called appressorium. Here, we found that protein MoAfo1(appressorium formation, MGG_10422) was involved in sensing signal molecules such as cutin monomers and long chain primary alcohols required for appressorium formation. The knockout mutant (ΔMoafo1) formed a few abnormal appressoria on the onion and rice sheath surfaces. However, it produced normal appressoria on the surface of rice leaves. MoAfo1 localized to the membranes of the cytoplasm and vacuole-like organelles in conidia and appressoria. Additionally, the ΔMoafo1 mutant showed defects in appressorium morphology, appressorium penetration, invasive growth, and pathogenicity. These multiple defects might be partially due to failure to respond properly to oxidative stress. These findings broaden our understanding of the fungal mechanisms at play in the recognition of the host surface during rice blast infection.

Mitogen-activated protein kinase cascade CgSte50-Ste11-Ste7-Mk1 regulates infection-related morphogenesis in the poplar anthracnose fungus Colletotrichum gloeosporioides

The hemibiotrophic pathogen Colletotrichum gloeosporioides is the causal agent of poplar anthracnose and causes considerable economic losses. This fungus infects its host through a specialized structure called an appressorium. In a previous study, we demonstrated that the mitogen-activated protein kinase (MAPK) CgMk1 plays a critical role in appressorium formation and pathogenicity. In this study, we identified three upstream components of CgMk1, the putative adaptor protein CgSte50, MAPKKK CgSte11, and MAPKK CgSte7, and showed that CgSte50, CgSte11, and CgSte7 positively regulate the phosphorylation of CgMk1. Deletion of CgSte50, CgSte11, and CgSte7 resulted in the loss of appressorium formation, penetration of the cellophane membrane, invasive growth and pathogenicity, similar to the defects observed in the CgMk1 mutant. CgSte50, CgSte11, CgSte7 and CgMk1 were also required for polarity during conidial germination. At the initial stage of appressorium formation, the accumulation of reactive oxygen species (ROS) was altered in the CgSte50, CgSte11, CgSte7 and CgMk1 deletion mutants compared with that in wild type (WT). Furthermore, the CgSte50, CgSte11, CgSte7 and CgMk1 deletion mutants manifested pleiotropic defects during vegetative growth; all mutants exhibited albino colonies, and the aerial hyphae had reduced hydrophobicity. In the mutants, autolysis was detected at the colony edge, and septum formation in the hyphae was elevated compared with that in the WT hyphae. Moreover, deletion of CgSte50, CgSte11, CgSte7 and CgMk1 affected vegetative growth under nitrogen-limiting and osmotic stress conditions. CgSte50, CgSte11, and CgSte7 but not CgMk1 were required for the oxidative stress response. Taken together, these results indicate that the CgMk1 MAPK cascade plays vital roles in various important functions in C. gloeosporioides.

The Redox Proteome of Thiol Proteins in the Rice Blast Fungus Magnaporthe oryzae

Redox modification, a post-translational modification, has been demonstrated to be significant for many physiological pathways and biological processes in both eukaryotes and prokaryotes. However, little is known about the global profile of protein redox modification in fungi. To explore the roles of redox modification in the plant pathogenic fungi, a global thiol proteome survey was performed in the model fungal pathogen Magnaporthe oryzae. A total of 3713 redox modification sites from 1899 proteins were identified through a mix sample containing mycelia with or without oxidative stress, conidia, appressoria, and invasive hyphae of M. oryzae. The identified thiol-modified proteins were performed with protein domain, subcellular localization, functional classification, metabolic pathways, and protein–protein interaction network analyses, indicating that redox modification is associated with a wide range of biological and cellular functions. These results suggested that redox modification plays important roles in fungal growth, conidium formation, appressorium formation, as well as invasive growth. Interestingly, a large number of pathogenesis-related proteins were redox modification targets, suggesting the significant roles of redox modification in pathogenicity of M. oryzae. This work provides a global insight into the redox proteome of the pathogenic fungi, which built a groundwork and valuable resource for future studies of redox modification in fungi.

Pathogenicity of Metarhizium rileyi against Spodoptera litura larvae: Appressorium differentiation, proliferation in hemolymph, immune interaction, and reemergence of mycelium

The pathogenicity of Metarhizium rileyi is a multi-faceted process that depends on many factors. This study attempts to decipher those factors of M. rileyi by investigating its pathogenicity against Spodoptera litura (Lepidoptera: Noctuidae) larvae. Through morphogenesis analysis, we for the first time demonstrated the infection structure, appressorium, of M. rileyi that can generate a more than 4 MPa turgor pressure. The Mrpmk1 gene was found to be essential for appressorium differentiation and mycelium reemerging, ΔMrpmk1 mutant exhibited no pathogenicity towards S. litura by natural infection process. Delayed appressorium formation time, decreased appressorium formation rate and turgor pressure of ΔMrpbs2 mutant manifested itself in postponed death time and lower mortality against S. litura. Following invasion into the larval hemocoel, M. rileyi cells transformed into blastospores, which may be conducive to dispersal and propagation, moreover, the blastospore form M. rileyi may subverted phagocytic defenses. Then M. rileyi cells morphed into extended hyphal body to cope with elongated hemocytes that participated in encapsulation. In the end, M. rileyi mycelia reemerged from the larval cadaver evenly to form muscardine cadaver. Eventually, conidia were produced to complete the infection cycle. During the infection, M. rileyi triggered both cellular and humoral immunity of S. litura. Besides morphological changes, stage-specifically produced oxalic acid and F-actin arrangement may play roles in nutrient acquisition and mycelium reemerging, respectively.

The GPI-Anchored GH76 Protein Dfg5 Affects Hyphal Morphology and Osmoregulation in the Mycoparasite Trichoderma atroviride and Is Interconnected With MAPK Signaling

The fungal cell wall is composed of a cross-linked matrix of chitin, glucans, mannans, galactomannans, and cell wall proteins with mannan chains. Cell wall mannans are directly attached to the cell wall core, while the majority of mannoproteins is produced with a glycosylphosphatidylinositol (GPI) anchor and then transferred to β-1,6-glucan in the cell wall. In this study, we functionally characterized the transmembrane protein Dfg5 of the glycoside hydrolase family 76 (GH76) in the fungal mycoparasite Trichoderma atroviride, whose ortholog has recently been proposed to cross-link glycoproteins into the cell wall of yeast and fungi. We show that the T. atroviride Dfg5 candidate is a GPI-anchored, transmembrane, 6-hairpin member of the GH76 Dfg5 subfamily that plays an important role in hyphal morphology in this mycoparasite. Alterations in the release of proteins associated with cell wall remodeling as well as a higher amount of non-covalently bonded cell surface proteins were detected in the mutants compared to the wild-type. Gene expression analysis suggests that transcript levels of genes involved in glucan synthesis, of proteases involved in mycoparasitism, and of the Tmk1 mitogen-activated protein kinase (MAPK)-encoding gene are influenced by Dfg5, whereas Tmk3 governs Dfg5 transcription. We show that Dfg5 controls important physiological properties of T. atroviride, such as osmotic stress resistance, hyphal morphology, and cell wall stability.

Cyclophilin BcCyp2 Regulates Infection-Related Development to Facilitate Virulence of the Gray Mold Fungus Botrytis cinerea

Cyclophilin (Cyp) and Ca²⁺/calcineurin proteins are cellular components related to fungal morphogenesis and virulence; however, their roles in mediating the pathogenesis of Botrytis cinerea, the causative agent of gray mold on over 1000 plant species, remain largely unexplored. Here, we show that disruption of cyclophilin gene BcCYP2 did not impair the pathogen mycelial growth, osmotic and oxidative stress adaptation as well as cell wall integrity, but delayed conidial germination and germling development, altered conidial and sclerotial morphology, reduced infection cushion (IC) formation, sclerotial production and virulence. Exogenous cyclic adenosine monophosphate (cAMP) rescued the deficiency of IC formation of the ∆Bccyp2 mutants, and exogenous cyclosporine A (CsA), an inhibitor targeting cyclophilins, altered hyphal morphology and prevented host-cell penetration in the BcCYP2 harboring strains. Moreover, calcineurin-dependent (CND) genes are differentially expressed in strains losing BcCYP2 in the presence of CsA, suggesting that BcCyp2 functions in the upstream of cAMP- and Ca²⁺/calcineurin-dependent signaling pathways. Interestingly, during IC formation, expression of BcCYP2 is downregulated in a mutant losing BcJAR1, a gene encoding histone 3 lysine 4 (H3K4) demethylase that regulates fungal development and pathogenesis, in B. cinerea, implying that BcCyp2 functions under the control of BcJar1. Collectively, our findings provide new insights into cyclophilins mediating the pathogenesis of B. cinerea and potential targets for drug intervention for fungal diseases.

Histone modification dynamics at H3K27 are associated with altered transcription of in planta induced genes in Magnaporthe oryzae

Transcriptional dynamic in response to environmental and developmental cues are fundamental to biology, yet many mechanistic aspects are poorly understood. One such example is fungal plant pathogens, which use secreted proteins and small molecules, termed effectors, to suppress host immunity and promote colonization. Effectors are highly expressed in planta but remain transcriptionally repressed ex planta, but our mechanistic understanding of these transcriptional dynamics remains limited. We tested the hypothesis that repressive histone modification at H3-Lys27 underlies transcriptional silencing ex planta, and that exchange for an active chemical modification contributes to transcription of in planta induced genes. Using genetics, chromatin immunoprecipitation and sequencing and RNA-sequencing, we determined that H3K27me3 provides significant local transcriptional repression. We detail how regions that lose H3K27me3 gain H3K27ac, and these changes are associated with increased transcription. Importantly, we observed that many in planta induced genes were marked by H3K27me3 during axenic growth, and detail how altered H3K27 modification influences transcription. ChIP-qPCR during in planta growth suggests that H3K27 modifications are generally stable, but can undergo dynamics at specific genomic locations. Our results support the hypothesis that dynamic histone modifications at H3K27 contributes to fungal genome regulation and specifically contributes to regulation of genes important during host infection.

Fungal pathogens of crops and humans pose annual threats to our food and health. There are many steps to the host infection process, during which fungal pathogens display unique growth, and use specific genes to cause disease. Despite this knowledge, many aspects of how pathogens regulate their genome to enact this process remain unknown. Here, we demonstrate how chemical modification of lysine residues on the histone H3, which helps organize and control DNA usage, play an important regulatory role in the model fungal pathogen causing rice blast disease. Our analysis shows a significant association between genes important for host infection and H3 lysine 27 methylation. We show that by experimentally changing histone modifications, many fungal genes normally used during plant infection are turned on outside of the host. Furthermore, we detail how histone modifications can change naturally in the fungus during plant infection. These findings help broaden our knowledge of genome regulation for these pathogens, and advances the goal of a more comprehensive understanding of the infection process.

Application of UPLC-QTOF-MS Based Untargeted Metabolomics in Identification of Metabolites Induced in Pathogen-Infected Rice

Metabolomics is a useful tool for comparing metabolite changes in plants. Because of its high sensitivity, metabolomics combined with high-resolution mass spectrometry (HR-MS) is the most widely accepted metabolomics tools. In this study, we compared the metabolites of pathogen-infected rice (Oryza sativa) with control rice using an untargeted metabolomics approach. We profiled the mass features of two rice groups using a liquid chromatography quadrupole time-of-flight mass spectrometry (QTOF-MS) system. Twelve of the most differentially induced metabolites in infected rice were selected through multivariate data analysis and identified through a mass spectral database search. The role of these compounds in metabolic pathways was finally investigated using pathway analysis. Our study showed that the most frequently induced secondary metabolites are prostanoids, a subclass of eicosanoids, which are associated with plant defense metabolism against pathogen infection. Herein, we propose a new untargeted metabolomics approach for understanding plant defense system at the metabolic level.

The Biology of Invasive Growth by the Rice Blast Fungus Magnaporthe oryzae

This introductory chapter describes the life cycle of Magnaporthe oryzae, the causal agent of rice blast disease. During plant infection, M. oryzae forms a specialized infection structure called an appressorium, which generates enormous turgor, applied as a mechanical force to breach the rice cuticle. Appressoria form in response to physical cues from the hydrophobic rice leaf cuticle and nutrient availability. The signaling pathways involved in perception of surface signals are described and the mechanism by which appressoria function is also introduced. Re-polarization of the appressorium requires a septin complex to organize a toroidal F-actin network at the base of the cell. Septin aggregation requires a turgor-dependent sensor kinase, Sln1, necessary for re-polarization of the appressorium and development of a rigid penetration hypha to rupture the leaf cuticle. Once inside the plant, the fungus undergoes secretion of a large set of effector proteins, many of which are directed into plant cells using a specific secretory pathway. Here they suppress plant immunity, but can also be perceived by rice immune receptors, triggering resistances. M. oryzae then manipulates pit field sites, containing plasmodesmata, to facilitate rapid spread from cell to cell in plant tissue, leading to disease symptom development.

PoRal2 Is Involved in Appressorium Formation and Virulence via Pmk1 MAPK Pathways in the Rice Blast Fungus Pyricularia oryzae

Pyricularia oryzae is an important plant pathogenic fungus that can severely damage rice and wheat crops, leading to significant reductions in crop productivity. To penetrate into and invade tissues of its plant host, this fungus relies on an invasive structure known as an appressorium. Appressorium formation is rigorously regulated by the cAMP-PKA and Pmk1 MAPK pathways. Here, we identified PoRal2, a homologous protein of Schizosaccharomyces pombe Ral2, and characterized its roles in fungal development and virulence in P. oryzae. PoRal2 contains N-terminal kelch repeats and C-terminal BTB domains. PoRal2 is involved in sporulation, aerial hypha and conidiophore differentiation, appressorium formation, plant penetration, and virulence. During appressorium formation, ∆Poral2 mutants generate appressoria with long germ tubes on hydrophobic surfaces. ∆Poral2 mutants exhibited a defective response to exogenous cAMP and the activated RAS2 ^G18V on a hydrophilic surface, indicating impairment in the cAMP-PKA or Pmk1 MAPK signaling pathways. Deletion of PoRAL2 leads to lowered Pmk1 phosphorylation level in the mutant. Moreover, PoRal2 is found to interact with Scd1, Smo1, and Mst50, which are involved in activation of Pmk1. In addition, the expression levels of MPG1, WISH, and PDEH in the cAMP-PKA pathway, RAS2 in both the cAMP-PKA and Pmk1 MAPK pathways, and melanin biosynthesis genes (ALB1, BUF1, and RSY1) were significantly down-regulated in the ∆Poral2. Therefore, PoRal2 is involved in fungal development and virulence by its crosstalk in the cAMP-PKA and Pmk1 MAPK signaling pathways.

Breakpoint: Cell Wall and Glycoproteins and their Crucial Role in the Phytopathogenic Fungi Infection

The cell wall that surrounds fungal cells is essential for their survival, provides protection against physical and chemical stresses, and plays relevant roles during infection. In general, the fungal cell wall is composed of an outer layer of glycoprotein and an inner skeletal layer of β-glucans or α- glucans and chitin. Chitin synthase genes have been shown to be important for septum formation, cell division and virulence. In the same way, chitin can act as a potent elicitor to activate defense response in several plant species; however, the fungi can convert chitin to chitosan during plant infection to evade plant defense mechanisms. Moreover, α-1,3-Glucan, a non-degradable polysaccharide in plants, represents a key feature in fungal cell walls formed in plants and plays a protective role for this fungus against plant lytic enzymes. A similar case is with β-1,3- and β-1,6-glucan which are essential for infection, structure rigidity and pathogenicity during fungal infection. Cell wall glycoproteins are also vital to fungi. They have been associated with conidial separation, the increase of chitin in conidial cell walls, germination, appressorium formation, as well as osmotic and cell wall stress and virulence; however, the specific roles of glycoproteins in filamentous fungi remain unknown. Fungi that can respond to environmental stimuli distinguish these signals and relay them through intracellular signaling pathways to change the cell wall composition. They play a crucial role in appressorium formation and penetration, and release cell wall degrading enzymes, which determine the outcome of the interaction with the host. In this review, we highlight the interaction of phypatophogen cell wall and signaling pathways with its host and their contribution to fungal pathogenesis.

Novel Fus3- and Ste12-interacting protein FsiA activates cell fusion-related genes in both Ste12-dependent and -independent manners in Ascomycete filamentous fungi

Filamentous fungal cells, unlike yeasts, fuse during vegetative growth. The orthologs of mitogen-activated protein (MAP) kinase Fus3 and transcription factor Ste12 are commonly involved in the regulation of cell fusion. However, the specific regulatory mechanisms underlying cell fusion in filamentous fungi have not been revealed. In the present study, we identified the novel protein FsiA as an AoFus3- and AoSte12-interacting protein in the filamentous fungus Aspergillus oryzae. The expression of AonosA and cell fusion-related genes decreased upon fsiA deletion and increased with fsiA overexpression, indicating that FsiA is a positive regulator of cell fusion. In addition, the induction of cell fusion-related genes by fsiA overexpression was also observed in the Aoste12 deletion mutant, indicating that FsiA can induce the cell fusion-related genes in an AoSte12-independent manner. Surprisingly, the fsiA and Aoste12 double deletion mutant exhibited higher cell fusion efficiency and increased mRNA levels of the cell fusion-related genes as compared to the fsiA single deletion mutant, which revealed that AoSte12 represses the cell fusion-related genes in the fsiA deletion mutant. Taken together, our data demonstrate that FsiA activates the cell fusion-related genes by suppressing the negative function of AoSte12 as well as by an AoSte12-independent mechanism.

Fatty Acid Synthase Beta Dehydratase in the Lipid Biosynthesis Pathway Is Required for Conidiogenesis, Pigmentation and Appressorium Formation in Magnaporthe oryzae S6

Lipid biosynthesis produces glycerol, which is important in fueling turgor pressure necessary for germination and penetration of plant host by fungi. As the relationship between pathogenicity and the lipid biosynthetic pathway is not fully understood, we have elucidated the role of the fatty acid synthase beta subunit dehydratase (FAS1) gene in lipid biosynthesis. The FAS1 gene was silenced through homologous double crossover in Magnaporthe oryzae strain S6 to study the effect on lipid biosynthesis. The vegetative growth of Δfas1 mutants show the highest drop on oleic acid (between 10 and 50%), while the mycelial dry weight of mutants dropped significantly on all media. Conidiation of FAS1 mutants show a ~10- and ~5-fold reduction on oatmeal and Potato Dextrose Agar (PDA), respectively. Mutants formed mycelium that were mildly pigmented, indicating that the deletion of FAS1 may have affected melanin biosynthesis. Biochemical and gene expression studies concluded that the fatty acid degradation pathway might have been interrupted by FAS1 deletion. FAS1 mutants showed no enzyme activity on glucose or olive oil, suggesting that the mutants may lack functional peroxisomes and be defective in β-oxidation of fatty acids, hence explaining the reduced lipid deposits in the spores.

The pheromone response module, a mitogen-activated protein kinase pathway implicated in the regulation of fungal development, secondary metabolism and pathogenicity

Mitogen-activated protein kinase (MAPK) pathways are highly conserved from yeast to human and are required for the regulation of a multitude of biological processes in eukaryotes. A pentameric MAPK pathway known as the Fus3 pheromone module was initially characterised in Saccharomyces cerevisiae and was shown to regulate cell fusion and sexual development. Individual orthologous pheromone module genes have since been found to be highly conserved in fungal genomes and have been shown to regulate a diverse array of cellular responses, such as cell growth, asexual and sexual development, secondary metabolite production and pathogenicity. However, information regarding the assembly and structure of orthologous pheromone modules, as well as the mechanisms of signalling and their biological significance is limited, specifically in filamentous fungal species. Recent studies have provided insight on the utilization of the pheromone module as a central signalling hub for the co-ordinated regulation of fungal development and secondary metabolite production. Various proteins of this pathway are also known to regulate reproduction and virulence in a range of plant pathogenic fungi. In this review, we discuss recent findings that help elucidate the structure of the pheromone module pathway in a myriad of fungal species and its implications in the control of fungal growth, development, secondary metabolism and pathogenicity.

MoFap7, a ribosome assembly factor, is required for fungal development and plant colonization of Magnaporthe oryzae

Fap7, an important ribosome assembly factor, plays a vital role in pre-40S small ribosomal subunit synthesis in Saccharomyces cerevisiae via its ATPase activity. Currently, the biological functions of its homologs in filamentous fungi remain elusive. Here, MoFap7, a homologous protein of ScFap7, was identified in the rice blast fungus Magnaporthe oryzae, which is a devastating fungal pathogen in rice and threatens food security worldwide. ΔMofap7 mutants exhibited defects in growth and development, conidial morphology, appressorium formation and infection, and were sensitive to oxidative stress. In addition, site-directed mutagenesis analysis confirmed that the conserved Walker A motif and Walker B motif in MoFap7 are essential for the biological functions of M. oryzae. We further analyzed the regulation mechanism of MoFap7 in pathogenicity. MoFap7 was found to interact with MoMst50, a regulator functioning in the MAPK Pmk1 signaling pathway, that participates in modulating plant penetration and cell-to-cell invasion by regulating the phosphorylation of MoPmk1. Moreover, MoFap7 interacted with the GTPases MoCdc42 and MoRac1 to control growth and conidiogenesis. Taken together, the results of this study provide novel insights into MoFap7-mediated orchestration of the development and pathogenesis of filamentous fungi.

Identification of a key G-protein coupled receptor in mediating appressorium formation and fungal virulence against insects

Fungal G-protein coupled receptors (GPCRs) play essential roles in sensing environmental cues including host signals. The study of GPCR in mediating fungus-insect interactions is still limited. Here we report the evolution of GPCR genes encoded in the entomopathogenic Metarhizium species and found the expansion of Pth11-like GPCRs in the generalist species with a wide host range. By deletion of ten candidate genes MrGpr1-MrGpr10 selected from the six obtained subfamilies in the generalist M. robertsii, we found that each of them played a varied level of roles in mediating appressorium formation. In particular, deletion of MrGpr8 resulted in the failure of appressorium formation on different substrates and the loss of virulence during topical infection of insects but not during injection assays when compared with the wild-type (WT) strain. Further analysis revealed that disruption of MrGpr8 substantially impaired the nucleus translocation of the mitogen-activated protein kinase (MAPK) Mero-Fus3 but not the MAPK Mero-Slt2 during appressorium formation. We also found that the defect of AMrGpr8 could not be rescued with the addition of cyclic AMP for appressorium formation. Relative to the WT, differential expression of the selected genes have also been detected in AMrGpr8. The results of this study may benefit the understanding of fungus-interactions mediated by GPCRs.

Perilipin LDP1 coordinates lipid droplets formation and utilization for appressorium-mediated infection in Magnaporthe oryzae

Lipid droplets (LDs) serve as one of the major reservoirs in conidia of Magnaporthe oryzae and are quickly utilized during appressorium formation. Here, we identified a gene, LDP1, encoding a perilipin that is important for LD formation and utilization during appressorium maturation. LDP1 is highly expressed in conidium and immature appressorium. Disruption mutants of LDP1 were significantly reduced in virulence, due to appressorial turgor reduction and difficulty in penetration. LDs were significantly reduced in the Δldp1 mutant, indicating LDP1 was required for LDs formation. LDP1 was colocalized with the LDs in conidium and immature appressorium but was gradually separated during appressorium maturation. A typical intracellular triacylglycerol lipase, TGL1-2, was clearly separated with LDs in conidium and immature appressorium but was well colocalized with LDs during appressorium maturation. The subcellular localization of TGL1-2 was affected by LDP1. These data suggested that LDP1 was bound to LDs for protecting from utilization in conidia and at the early appressorium stage but was separated from LDs for lipase entering and degradation. LDP1 was phosphorylated by CPKA at Thr96, which was essential for its localization and functions. These data indicate perilipin LDP1 can coordinate LD formation and utilization for appressorium-mediated infection of M. oryzae.

The phosphatase Ptc6 is involved in virulence and MAPK signalling in Fusarium oxysporum

Mitogen-activated kinase (MAPK) signalling pathways are involved in several important processes related to the development and virulence of Fusarium oxysporum. Reversible phosphorylation of the protein members of these pathways is a major regulator of essential biological processes. Among the phosphatases involved in dephosphorylation of MAPKs, type 2C protein phosphatases (PP2Cs) play important roles regulating many developmental strategies and stress responses in yeasts. Nevertheless, the PP2C family is poorly known in filamentous fungi. The F. oxysporum PP2C family includes seven proteins, but only Ptc1 has been studied so far. Here we show the involvement of Ptc6 in the stress response and virulence of F. oxysporum. Expression analysis revealed increased expression of ptc6 in response to cell wall and oxidative stresses. Additionally, targeted inactivation of ptc6 entailed enhanced susceptibility to cell wall stresses caused by Calcofluor White (CFW). We also demonstrate that the lack of Ptc6 deregulates both the Mpk1 phosphorylation induced by CFW and, more importantly, the Fmk1 dephosphorylation induced by pH acidification of the extracellular medium, indicating that Ptc6 is involved in the regulation of these MAPKs. Finally, we showed, for the first time, the involvement of a phosphatase in the invasive growth and virulence of F. oxysporum.

The C3HC type zinc-finger protein (ZFC3) interacting with Lon/MAP1 is important for mitochondrial gene regulation, infection hypha development and longevity of Magnaporthe oryzae

Background

The rice blast is a typical fungal disease caused by Magnaporthe oryzae, and the mitochondrial ATP-dependent Lon protease (MAP1) has been proven to be involved in blast development. We previously screened a C3HC type Zinc-finger domain protein (ZFC3), which is interacted with MAP1. The purpose of this research was to study the biological function of ZFC3 protein in M. oryzae.

Results

We first confirmed that the ZFC3-RFP fusion protein is localized within the mitochondria. The deleted mutant strains of ZFC3 (∆ZFC3) showed the enhanced expression level of mtATP6, particularly mtATP8, and almost unchanged nATP9. ΔZFC3 produces more conidia and more tolerance to multiple stressors. The knock-out strain shows more melanin accumulation suggests the susceptibility to aging. ΔZFC3 displays faster early-stage hypha infiltration involved in MAP1-mediated pathogenicity in host rice.

Conclusion

These results support the view that ZFC3 is a key regulator involved in gene regulation, stress response, cell wall integrity, longevity, conidiation, infection hypha development and MAP1-mediated pathogenicity in M. oryzae.

Comparative genomic and transcriptomic analyses reveal different pathogenicity-related genes among three eucalyptus fungal pathogens

Ceratocystis fimbriata is an important plant pathogen known to cause Ceratocystis Wilt (CW), a prevalent fungal disease known to affect Eucalyptus spp. plantations in Brazil. To better understand the molecular mechanisms related to pathogenicity in eucalyptus, we generated a high-quality assembly and annotation of the Ce. fimbriata LPF1912 isolate (LPF1912) genome, as well as the first transcriptome of LPF1912 from 16 eucalyptus clones at three infection incubation periods (12, 18, and 24 h). The LPF1912 genome assembly contains 805 scaffolds, totaling 31.8 Mb, with 43% of the genome estimated to be coding sequence comprised of 7,390 protein-coding genes of which 626 (8.5%) were classified as secreted proteins, 120 ribosomal RNAs, and 532 transfer RNAs. Comparative genomic analysis among three eucalyptus fungal pathogens (Ce. fimbriata, Ce. eucalypticola, and Calonectria pseudoreteaudii), showed high similarity in the proteome (21.81%) and secretome (52.01%) of LPF1912 and Ce. eucalypticola. GO annotation of pathogenicity-related genes of LPF1912 and Ce. eucalypticola, revealed enrichment in cell wall degrading enzymes (CWDEs), and lipid/cutin metabolism for Ca. pseudoreteaudii. Additionally, a transcriptome analysis between resistant and susceptible eucalyptus clones to CW infection indicated that a majority (11) of LPF1912 differentially expressed genes had GO terms associated with enzymatic functions, such as the polygalacturonase gene family, confirming the crucial role of CWDEs for Ce. fimbriata pathogenicity. Finally, our genomic and transcriptomic analysis approach provides a better understanding of the mechanisms involved in Ce. fimbriata pathogenesis, as well as a framework for further studies.

The H3K4 demethylase Jar1 orchestrates ROS production and expression of pathogenesis-related genes to facilitate Botrytis cinerea virulence

Histone 3 Lysine 4 (H3K4) demethylation is ubiquitous in organisms, however the roles of H3K4 demethylase JARID1(Jar1)/KDM5 in fungal development and pathogenesis remain largely unexplored. Here, we demonstrate that Jar1/KDM5 in Botrytis cinerea, the grey mould fungus, plays a crucial role in these processes. The BcJAR1 gene was deleted and its roles in fungal development and pathogenesis were investigated using approaches including genetics, molecular/cell biology, pathogenicity and transcriptomic profiling. BcJar1 regulates H3K4me3 and both H3K4me2 and H3K4me3 methylation levels during vegetative and pathogenic development, respectively. Loss of BcJAR1 impairs conidiation, appressorium formation and stress adaptation; abolishes infection cushion (IC) formation and virulence, but promotes sclerotium production in the ΔBcjar1 mutants. BcJar1 controls reactive oxygen species (ROS) production and proper assembly of Sep4, a core septin protein and virulence determinant, to initiate infection structure (IFS) formation and host penetration. Exogenous cAMP partially restored the mutant appressorium, but not IC, formation. BcJar1 orchestrates global expression of genes for ROS production, stress response, carbohydrate transmembrane transport, secondary metabolites, etc., which may be required for conidiation, IFS formation, host penetration and virulence of the pathogen. Our work systematically elucidates BcJar1 functions and provides novel insights into Jar1/KDM5-mediated H3K4 demethylation in regulating fungal development and pathogenesis.

The putative deubiquitinating enzyme MoUbp4 is required for infection-related morphogenesis and pathogenicity in the rice blast fungus Magnaporthe oryzae

Ubiquitination is a key regulatory mechanism that affects numerous important biological processes, including cellular differentiation and pathogenesis in eukaryotic cells. Attachment of proteins to ubiquitin is reversed by specialized proteases, deubiquitinating enzymes (DUBs), which are essential for precursor processing, maintaining ubiquitin homeostasis and promoting protein degradation by recycling ubiquitins. Here, we report the identification of a novel non-pathogenic T-DNA-tagged mutant T612 of Magnaporthe oryzae with a single insertion in the second exon of MoUBP4, which encodes a putative ubiquitin carboxyl-terminal hydrolase. Targeted gene deletion mutants of MoUBP4 are significantly reduced in mycelial growth, conidiation, and increased in tolerance to SDS and CR (Congo red) cell-wall damage. The ΔMoubp4 mutants are blocked in penetration and invasive growth, which results in the loss of pathogenicity. Many conidia produced by the ΔMoubp4 mutants are unable to form appressoria and mobilization and degradation of glycogen and lipid droplets are significantly delayed. Moreover, immunohybridization analysis revealed that total protein ubiquitination levels of the null mutants were significantly increased, indicating that MoUbp4 functions as a deubiquitination enzyme. Taken together, we conclude that MoUbp4 is required for deubiquitination, infection-related morphogenesis and pathogenicity in M. oryzae.

Extremely Low Concentrations of Acetic Acid Stimulate Cell Differentiation in Rice Blast Fungus

Metabolic switching and rewiring play a dynamic role in programmed cell differentiation. Many pathogenic microbes need to survive in nutrient-deficient conditions and use the glyoxylate cycle, an anaplerotic pathway of the tricarboxylic acid cycle, to produce carbohydrates. The plant pathogenic fungus Magnaporthe oryzae (Pyricularia oryzae) has a unique chitin deacetylase, Cbp1. The spatiotemporal activity of this protein is required for modification of the M. oryzae wall and for cell differentiation into the specialized infection structure (appressorium). Here we show that acetic acid, another product released by the Cbp1-catalyzed conversion of chitin into chitosan, induces appressorium formation. An extremely low concentration (fM) of acetic acid restored cell differentiation in a Δcbp1 mutant possibly through the glyoxylate cycle.

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Acidification occurred by chitin deacetylase activity during cell differentiation

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Extremely low concentrations of exogenous acetic acid stimulated cell differentiation

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Exogenous acetic acid induced ICL1 expression, a member of the glyoxylate cycle

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Deletion of ICL1 inhibited acetic acid-mediated cell differentiation

The Verticillium dahliae Sho1-MAPK pathway regulates melanin biosynthesis and is required for cotton infection

Verticillium dahliae is a soil‐borne fungus that causes vascular wilt on numerous plants worldwide. The fungus survives in the soil for up to 14 years by producing melanized microsclerotia. The protective function of melanin in abiotic stresses is well documented. Here, we found that the V. dahliae tetraspan transmembrane protein VdSho1, a homolog of the Saccharomyces cerevisiae Sho1, acts as an osmosensor, and is required for plant penetration and melanin biosynthesis. The deletion mutant ΔSho1 was incubated on a cellophane membrane substrate that mimics the plant epidermis, revealing that the penetration of ΔSho1 strain was reduced compared to the wild‐type strain. Furthermore, VdSho1 regulates melanin biosynthesis by a signalling mechanism requiring a kinase‐kinase signalling module of Vst50‐Vst11‐Vst7. Strains, ΔVst50, ΔVst7 and ΔVst11 also displayed defective penetration and melanin production like the ΔSho1 strain. Defects in penetration and melanin production in ΔSho1 were restored by overexpression of Vst50, suggesting that Vst50 lies downstream of VdSho1 in the regulatory pathway governing penetration and melanin biosynthesis. Data analyses revealed that the transmembrane portion of VdSho1 was essential for both membrane penetration and melanin production. This study demonstrates that Vst50‐Vst11‐Vst7 module regulates VdSho1‐mediated plant penetration and melanin production in V. dahliae, contributing to virulence.

Detection of putative pathogenicity and virulence genes of Erysiphe pisi using genome-wide in-silico search and their suppression by er2 mediated resistance in garden pea

The biotrophic fungus, Erysiphe pisi is the chief causal agent of powdery mildew disease of garden pea. A genome-wide search using in-silico approach was carried to detect putative pathogenicity and virulence genes of E. pisi, since information about these genes and their interaction with pea is limited. Nineteen putative pathogenicity gene sequences were detected through genome-wide pathogenicity gene-search and confirmed them to be conserved in E. pisi through genomic PCRs. Fifteen of these genes expressed through reverse transcriptase-polymerase chain reaction (RT-PCR) amplifying expected band size along with fungal and plant specific internal controls. Gene sequencing and annotation revealed them to be Erysiphe-specific. A time course study was carried to monitor expression of nine of these genes through real-time quantitative (qRT)-PCR in Erysiphe-challenged plants of powdery mildew resistant pea genotype, JI-2480 carrying er2 gene and susceptible pea cultivar, Arkel. Expression of these genes was differentially and temporally regulated. They were found mostly related to signaling; cAMP-PKA (cPKA, CRP and AC) and MAPK (MST7) pathways along with MFP, TRE and PEX which are reported pathogenicity factors in other ascomycete members indicating that similar conserved pathways function in E. pisi also. These genes expressed at higher level at initial hours post inoculation (hpi) as early as 6 hpi in Arkel compared to JI-2480 implying them as pathogenicity factors. The elevated level of expression of MFP, TRE, CRP and cPKA gene sequences in E. pisi-challenged JI-2480 genotype at 12 hpi alone suggests these genes to possess a role in avirulence in JI-2480, conferring er2 mediated resistance.

Rapid adaptation of signaling networks in the fungal pathogen Magnaporthe oryzae

Background

One fundamental question in biology is how the evolution of eukaryotic signaling networks has taken place. “Loss of function” (lof) mutants from components of the high osmolarity glycerol (HOG) signaling pathway in the filamentous fungus Magnaporthe oryzae are viable, but impaired in osmoregulation.

Results

After long-term cultivation upon high osmolarity, stable individuals with reestablished osmoregulation capacity arise independently from each of the mutants with inactivated HOG pathway. This phenomenon is extremely reproducible and occurs only in osmosensitive mutants related to the HOG pathway – not in other osmosensitive Magnaporthe mutants. The major compatible solute produced by these adapted strains to cope with high osmolarity is glycerol, whereas it is arabitol in the wildtype strain. Genome and transcriptome analysis resulted in candidate genes related to glycerol metabolism, perhaps responsible for an epigenetic induced reestablishment of osmoregulation, since these genes do not show structural variations within the coding or promotor sequences.

Conclusion

This is the first report of a stable adaptation in eukaryotes by producing different metabolites and opens a door for the scientific community since the HOG pathway is worked on intensively in many eukaryotic model organisms.

Metarhizium robertsii ammonium permeases (MepC and Mep2) contribute to rhizoplane colonization and modulates the transfer of insect derived nitrogen to plants

The endophytic insect pathogenic fungi (EIPF) Metarhizium promotes plant growth through symbiotic association and the transfer of insect-derived nitrogen. However, little is known about the genes involved in this association and the transfer of nitrogen. In this study, we assessed the involvement of six Metarhizium robertsii genes in endophytic, rhizoplane and rhizospheric colonization with barley roots. Two ammonium permeases (MepC and Mep2) and a urease, were selected since homologous genes in arbuscular mycorrhizal fungi were reported to play a pivotal role in nitrogen mobilization during plant root colonization. Three other genes were selected on the basis on RNA-Seq data that showed high expression levels on bean roots, and these encoded a hydrophobin (Hyd3), a subtilisin-like serine protease (Pr1A) and a hypothetical protein. The root colonization assays revealed that the deletion of urease, hydrophobin, subtilisin-like serine protease and hypothetical protein genes had no impact on endophytic, rhizoplane and rhizospheric colonization at 10 or 20 days. However, the deletion of MepC resulted in significantly increased rhizoplane colonization at 10 days whereas ΔMep2 showed increased rhizoplane colonization at 20 days. In addition, the nitrogen transporter mutants also showed significantly higher 15N incorporation of insect derived nitrogen in barley leaves in the presence of nutrients. Insect pathogenesis assay revealed that disruption of MepC, Mep2, urease did not reduce virulence toward insects. The enhanced rhizoplane colonization of ΔMep2 and ΔMepC and insect derived nitrogen transfer to plant hosts suggests the role of MepC and Mep2 in Metarhizium-plant symbiosis.

System-Wide Characterization of MoArf GTPase Family Proteins and Adaptor Protein MoGga1 Involved in the Development and Pathogenicity of Magnaporthe oryzae

ADP ribosylation factor (Arf) small GTPase family members are involved in vesicle trafficking and organelle maintenance in organisms ranging from Saccharomyces cerevisiae to humans. A previous study identified Magnaporthe oryzae Arf6 (MoArf6) as one of the Arf proteins that regulates growth and conidiation in the rice blast fungus M. oryzae, but the remaining family proteins remain unknown. Here, we identified six additional Arf proteins, including MoArf1, MoArl1, MoArl3, MoArl8, MoCin4, and MoSar1, as well as their sole adaptor protein, MoGga1, and determined their shared and specific functions. We showed that the majority of these proteins exhibit positive regulatory functions, most notably, in growth. Importantly, MoArl1, MoCin4, and MoGga1 are involved in pathogenicity through the regulation of host penetration and invasive hyphal growth. MoArl1 and MoCin4 also regulate normal vesicle trafficking, and MoCin4 further controls the formation of the biotrophic interfacial complex (BIC). Moreover, we showed that Golgi-cytoplasm cycling of MoArl1 is required for its function. Finally, we demonstrated that interactions between MoArf1 and MoArl1 with MoGga1 are important for Golgi localization and pathogenicity. Collectively, our findings revealed the shared and specific functions of Arf family members in M. oryzae and shed light on how these proteins function through conserved mechanisms to govern growth, transport, and virulence of the blast fungus.IMPORTANCEMagnaporthe oryzae is the causal agent of rice blast, representing the most devastating diseases of rice worldwide, which results in losses of amounts of rice that could feed more than 60 million people each year. Arf (ADP ribosylation factor) small GTPase family proteins are involved in vesicle trafficking and organelle maintenance in eukaryotic cells. To investigate the function of Arf family proteins in M. oryzae, we systematically characterized all seven Arf proteins and found that they have shared and specific functions in governing the growth, development, and pathogenicity of the blast fungus. We have also identified the pathogenicity-related protein MoGga1 as the common adaptor of MoArf1 and MoArl1. Our findings are important because they provide the first comprehensive characterization of the Arf GTPase family proteins and their adaptor protein MoGga1 functioning in a plant-pathogenic fungus, which could help to reveal new fungicide targets to control this devastating disease.

A multi-omics analysis of the grapevine pathogen Lasiodiplodia theobromae reveals that temperature affects the expression of virulence- and pathogenicity-related genes

Lasiodiplodia theobromae (Botryosphaeriaceae, Ascomycota) is a plant pathogen and human opportunist whose pathogenicity is modulated by temperature. The molecular effects of temperature on L. theobromae are mostly unknown, so we used a multi-omics approach to understand how temperature affects the molecular mechanisms of pathogenicity. The genome of L. theobromae LA-SOL3 was sequenced (Illumina MiSeq) and annotated. Furthermore, the transcriptome (Illumina TruSeq) and proteome (Orbitrap LC-MS/MS) of LA-SOL3 grown at 25 °C and 37 °C were analysed. Proteins related to pathogenicity (plant cell wall degradation, toxin synthesis, mitogen-activated kinases pathway and proteins involved in the velvet complex) were more abundant when the fungus grew at 25 °C. At 37 °C, proteins related to pathogenicity were less abundant than at 25 °C, while proteins related to cell wall organisation were more abundant. On the other hand, virulence factors involved in human pathogenesis, such as the SSD1 virulence protein, were expressed only at 37 °C. Taken together, our results showed that this species presents a typical phytopathogenic molecular profile that is compatible with a hemibiotrophic lifestyle. We showed that L. theobromae is equipped with the pathogenesis toolbox that enables it to infect not only plants but also animals.

Two protein kinases UvPmk1 and UvCDC2 with significant functions in conidiation, stress response and pathogenicity of rice false smut fungus Ustilaginoidea virens

Ustilaginoidea virens is an important fungus causing rice false smut, a devastating disease on spikelets of rice. In this study, we identified and characterized two CMGC (CDK/MAPK/GSK3/CLK) kinase genes, UvPmk1 and UvCDC2, in U. virens. Although UvPmk1 and UvCDC2 are, respectively, homologous to Fus3/Kss1 mitogen-activated protein kinases (MAPKs) and cyclin-dependent kinases (CDKs), they all have a conserved serine/threonine protein kinase domain. The qRT-PCR analysis of the relative expression of UvPmk1 and UvCDC2 during the infection of U. virens showed that these two genes were highly expressed during infection. UvPmk1 and UvCDC2 knockout mutants exhibited no significant changes in mycelial vegetative growth but decreases in conidiation. In addition, both UvPmk1 and UvCDC2 knockout mutants showed increases in tolerance to hyperosmotic and cell wall stresses, but they, respectively, exhibited decreases and increases in tolerance to oxidative stress compared with the wild-type strain HWD-2. Pathogenicity and infection assays demonstrated the defective growth of infection hyphae and significant loss of virulence in UvPmk1 and UvCDC2 knockout mutants. Taken together, our results demonstrate that UvPmk1 and UvCDC2 play important roles in the conidiation, stress response, and pathogenicity of U. virens.

MoGT2 Is Essential for Morphogenesis and Pathogenicity of Magnaporthe oryzae

The ascomycete fungus Magnapothe oryzae is the causal agent of rice blast disease, leading to severe loss in cultivated rice production worldwide. In this study, we identified a conserved type 2 glycosyltransferase named MoGt2 in M. oryzae. The mogt2Δ targeted gene deletion mutants exhibited pleiotropic defects in vegetative growth, conidiation, stress response, hyphal appressorium-mediated penetration, and pathogenicity. Furthermore, conserved glycosyltransferase domains are critical for MoGt2 function. The comparative transcriptome analysis revealed potential target genes under MoGt2 regulation in M. oryzae conidiation. Identification of potential glycoproteins modified by MoGt2 provided information on its regulatory mechanism of gene expression and biological functions. Overall, our study represents the first report of type 2 glycosyltransferase function in M. oryzae infection-related morphogenesis and pathogenesis.

Magnaporthe oryzae causes the rice blast disease, which is one of the most serious diseases of cultivated rice worldwide. Glycosylation is an important posttranslational modification of secretory and membrane proteins in all eukaryotes, catalyzed by glycosyltransferases (GTs). In this study, we identified and characterized a type 2 glycosyltransferase, MoGt2, in M. oryzae. Targeted gene deletion mutants of MoGT2 (mogt2Δ strains) were nonpathogenic and were impaired in vegetative growth, conidiation, and appressorium formation at hyphal tips. Moreover, MoGT2 plays an important role in stress tolerance and hydrophobin function of M. oryzae. Site-directed mutagenesis analysis showed that conserved glycosyltransferase domains (DxD and QxxRW) are critical for biological functions of MoGt2. MoGT2 deletion led to altered glycoproteins during M. oryzae conidiation. By liquid chromatography-tandem mass spectrometry (LC-MS/MS), we identified several candidate proteins as potential substrates of MoGt2, including several heat shock proteins, two coiled-coil domain-containing proteins, aminopeptidase 2, and nuclease domain-containing protein 1. On the other hand, we found that a conidiation-related gene, genes involved in various metabolism pathways, and genes involved in cell wall integrity and/or osmotic response were differentially regulated in the mogt2Δ mutant, which may potentially contribute to its condiation defects. Taken together, our results show that MoGt2 is important for infection-related morphogenesis and pathogenesis in M. oryzae.

IMPORTANCE The ascomycete fungus Magnapothe oryzae is the causal agent of rice blast disease, leading to severe loss in cultivated rice production worldwide. In this study, we identified a conserved type 2 glycosyltransferase named MoGt2 in M. oryzae. The mogt2Δ targeted gene deletion mutants exhibited pleiotropic defects in vegetative growth, conidiation, stress response, hyphal appressorium-mediated penetration, and pathogenicity. Furthermore, conserved glycosyltransferase domains are critical for MoGt2 function. The comparative transcriptome analysis revealed potential target genes under MoGt2 regulation in M. oryzae conidiation. Identification of potential glycoproteins modified by MoGt2 provided information on its regulatory mechanism of gene expression and biological functions. Overall, our study represents the first report of type 2 glycosyltransferase function in M. oryzae infection-related morphogenesis and pathogenesis.

The farnesyltransferase β-subunit RAM1 regulates localization of RAS proteins and appressorium-mediated infection in Magnaporthe oryzae

Post-translational farnesylation can regulate subcellular localization and protein-protein interaction in eukaryotes. The function of farnesylation is not well identified in plant pathogenic fungi, particularly during the process of fungal infection. Here, through functional analyses of the farnesyltransferase β-subunit gene, RAM1, we examine the importance of protein farnesylation in the rice blast fungus Magnaporthe oryzae. Targeted disruption of RAM1 resulted in the reduction of hyphal growth and sporulation, and an increase in the sensitivity to various stresses. Importantly, loss of RAM1 also led to the attenuation of virulence on the plant host, characterized by decreased appressorium formation and invasive growth. Interestingly, the defect in appressoria formation of the Δram1 mutant can be recovered by adding exogenous cAMP and IBMX, suggesting that RAM1 functions upstream of the cAMP signalling pathway. We found that two Ras GTPases, RAS1 and RAS2, can interact with Ram1, and their plasma membrane localization was regulated by Ram1 through their C-terminal farnesylation sites. Adding a farnesyltransferase inhibitor Tipifarnib can result in similar defects as in Δram1 mutant, including decreased appressorium formation and invasive growth, as well as mislocalized RAS proteins. Our findings indicate that protein farnesylation regulates the RAS protein-mediated signaling pathways required for appressorium formation and host infection, and suggest that abolishing farnesyltransferase could be an effective strategy for disease control.

Analysis of Pyricularia oryzae and P. grisea from Different Hosts Based on Multilocus Phylogeny and Pathogenicity Associated with Host Preference in China

Pyricularia oryzae and P. grisea are important agents of major diseases on a wide range of gramineous hosts. Whereas P. oryzae is the most important pathogen causing outbreaks of rice blast, P. grisea is mainly a pathogen of crabgrass. In this study, 103 P. oryzae and 20 P. grisea isolates were collected from seven species of plants, and we analyzed their phylogeny, pathogenicity, and relationship with host preferences to investigate the differences among them from different hosts. Based on phylogenetic analysis of multilocus sequences, 16 isolates from crabgrass and four isolates from green bristlegrass were identified as P. grisea and another 103 isolates from crabgrass, green bristlegrass, goose grass, foxtail millet, wild millet, rice, and sedge belonged to P. oryzae. Results of pathogenicity tests by artificial inoculation demonstrated that six of 10 P. oryzae isolates from rice and three of 44 P. oryzae isolates from green bristlegrass showed cross-infectivity on green bristlegrass and rice, respectively. Taken together, our results demonstrated that isolates from green bristlegrass and crabgrass consist of both P. oryzae and P. grisea and that P. oryzae isolates showed cross-infectivity between rice and green bristlegrass, suggesting that host shifts may occur for P. oryzae and P. grisea.